Tempus Fugit
by Kerr Avonsen
Summary: Sometimes it takes a complete stranger to give you a new perspective.


**Tempus Fugit**

by Kathryn Andersen

**Fandom:** Doctor Who/Stargate SG1  
**Character(s):** Samantha Carter, Tenth Doctor  
**Warnings (if any):** none  
**Spoilers for:** Doctor Who - "Blink"; Stargate SG1 - "1969", "2010", "2001"  
**Notes:** The request was _Scientific accuracy meets "wibbly-wobbly timey-wimey stuff"_.  
Thanks to JB for brainstorming and alpha reading, JP for suggestions about dinner conversation, and BR and Reatha for beta reading.  
**Written for:** Spoofmaster  
**Words:** 1900  
**Summary:** Sometimes it takes a complete stranger to give you a new perspective.  
Written in 2008 for the dw_cross ficathon.

* * *

Sam studied the sugar packets in their pottery container as she waited for her dinner to arrive. Only one packet of white sugar left, three packets of raw sugar and about ten of non-sugar sweetener. Her fellow conference-goers obviously believed in real sucrose, not substitutes.

She'd wanted to attend this Astrophysics conference since it had been announced; it wasn't every day that a new solar body was discovered. She'd had to sell it to General Hammond as part of their cover - a deep space telemetry project would have to have some interest in deep space, after all - but she'd relished the opportunity to give a paper on Ixion nonetheless. And after what had happened with the Aschen, getting away from the SGC couldn't have happened at a better time. She could still remember falling through the Stargate, calling out for them to close the iris... leaving Joseph behind to die. What was the use of getting a warning from the future if it didn't enable you to save everybody?

"Doctor Carter?" an accented voice said.

"It's Major Carter, actually," she said automatically as she looked up. Unruly brown hair, thirties, not bad looking, brown suit. British, from the accent, and vaguely familiar.

He smiled. "I won't hold it against you. One of my best friends is a brigadier."

She placed the voice. "The albedo question, right?" He'd stuck in her mind as one of the few people there with a genuine question, interested in what she had to say, and not in redirecting the topic to their own area of research.

He beamed at her and sat down opposite her. "That's me." He grabbed a napkin and scribbled on it, still talking. "This is what I was referring to: if you consider the spectroanalysis further, then the composition of Ixion could indicate a different albedo, and-" He pushed the napkin over to her.

"A higher albedo would mean a lower mass, yes, but-" She considered the equations he'd written there. "But the spectroanalysis indicated that there's no water ice, which is the most likely thing to increase the albedo of a plutino."

"No, this doesn't assume water ice, just a mixture of dark carbon and tholin," he said.

By the time the waitress stopped to ask if she should set another place, the salt and pepper had been commandeered to represent Neptune and Pluto, a glass was the sun, bits of her bread roll were various trans-Neptunian objects, and three more napkins were covered with formulae written in two different hands.

"Would you like to join me?" Sam asked. It was invigorating to be so intellectually stimulated, to talk to someone who actually understood what she was talking about, and more, had something to contribute to the discussion.

"I'd be delighted, Major Carter."

"I didn't catch your name, Doctor...?" Sam said.

"Smith, John Smith," he said. "But people call me the Doctor."

"_The_ Doctor?" She supposed that with a name like John Smith, he'd rather use a nickname; that or he had a very large ego. But if he did, he was still a lot more charming than Rodney McKay.

"Yep." He smiled. "The Doctor, that's me."

**-oOo-**

"...but if light is either a particle or a wave..." Sam said, then took a forkful of her risotto.

"It isn't," the Doctor said.

"Yes, I mean, it's _both_," Sam said, waving her hands.

"No, it isn't either one," the Doctor said. "They're just the models you use to describe its behaviour."

**-oOo-**

"Suppose I were to write myself a note," Sam said, "and send it back in time-"

"You'd have to have a very good reason," the Doctor said.

"I remembered getting the note in the past and knew I'd have to send it to myself." It had been General Hammond who'd written the note to himself, of course, but she couldn't talk about it at all unless she pretended it was hypothetical. And maybe Dr. Smith would have some insights that would shake up her own tired circle of thoughts on the subject.

"Okay, that _might_ be a good reason," he said.

"But it's an ontological paradox," Sam protested.

"That's the safest kind," said the Doctor.

"Safest?"

"A closed loop is stable," the Doctor said. "Of course, from a non-linear, non-subjective viewpoint, it isn't actually a loop. More like a spiral. Well, not a spiral, exactly..." He picked up a squeeze-bottle of mustard, and drew a circle with it on his bread-plate. "You have a loop..." Then he picked up the bottle of ketchup and squeezed another circle, right on top of the mustard.

"And then another loop," Sam said, leaning forward. "But there was a first loop," she said. "The one where the information came from."

"Exactly," the Doctor said. "Once the loop is closed, the origin is lost, but that doesn't mean that there wasn't one."

"But doesn't that defy entropy?" Sam said.

"Objects are subject to entropy," the Doctor said. "Information isn't quite the same." He cut a piece off his steak and dipped it in the mustard-ketchup mixture on his bread plate. "Hmmm, American mustard is... hardly there, isn't it?" He chewed some more. "Say I'm trapped in 1969..."

She tried to look unconcerned. It was probably nothing. A coincidence. "Why 1969?"

"Dunno, I haven't made a study of the variable time displacement of Weeping Angels. Bit hard to collect the data safely. But not a bad year to be stuck in, really. I mean, the Moon Landing! Brilliant, that. Beans on toast is not so brilliant, but if you've got tea, well, everything is better with tea." He demonstrated this adage by taking a sip from his cup.

Sam took another bite of her salad, hardly tasting it. "So you're stuck in 1969..." She was right about him shaking up her thoughts; he might be a little crazy, but he had her complete attention.

"Yes, right." The Doctor sat up straighter. "And I have a folder of information which was handed to me in, say, 2007, by a complete stranger, let's call her Sally. Information I need in order to be able to send a message to Sally in 2006 so that she can send my time machine back to me in 1969."

Definitely a coincidence, then. Just a hypothetical; because there were no such things as time machines; not ones that could be _sent_ somewhere. Her own encounters with time travel had required a Stargate. "That's a closed loop."

"Right," the Doctor said. "But the folder contains a letter and photographs that were given to Sally by the grandson of Kathy. If I gave the letter and photographs to Kathy to give to her grandson, that would be a problem. But if I just persuaded Kathy to write a letter to give to her grandson, in which she enclosed photographs she'd already taken, then it's a movement of information, rather than of objects."

"All right," Sam said, "suppose I got a note from the future, that told me not to do something, so I didn't do it." The note with Jack's blood on it; the note that had said too much and not enough.

"That's not a closed loop."

"No," Sam said. "But then say I do something else that nearly leads to the same outcome, but manage to avoid the danger because the note made me suspicious." But not suspicious enough, not soon enough.

"History doesn't like being changed," the Doctor said with a frown. "It tends to want to go back on the same track."

"How can that be?" Sam said. "Either you can change history, or you can't, surely?"

"Time is like... molten quartz," the Doctor said. "At a point, it is crystallized, solid. You might call that point the Present. Ahead of the wavefront of the crystallized Present, time is mutable to a greater or lesser degree. But travel back into Deep Time is impossible. Try to change that Past, and Time shatters. Actually, it already has."

"You're acting as if time travel is actually possible."

"You know it is," he said. "You've travelled in time yourself."

The last forkful of risotto turned to ashes in her mouth. Was there a leak? "And you know this because...?"

"Time travel leaves traces," he said.

Was he a spy, or did he really know that? Time to call his bluff. "If you know that much," Sam said, looking him squarely in the eyes, "how much more do you know?"

He opened up one of the packets of sugar and poured it on the table. "I know that I can't predict where each grain is going to fall."

"You could sprinkle the sugar, see how it comes out, and come back and tell me," said Sam. "Then you _could_ predict it."

"All right, now you try it," the Doctor said.

Sam picked up the last packet of sugar, tore it open, tipped it out - but the Doctor's hand was in the way, capturing the sugar before it hit the table.

Then he turned his hand over, and let the sugar fall on the table. "I could interfere with your choice to pour out the sugar... but I let the sugar fall anyway. So did I override your choice? Or was I the means by which you made it happen?"

"It's not all as simple as a packet of sugar," she said.

"Exactly," he said.

She looked down at the remnants of her salad. Tossed, bits and pieces, chaos. "Too many variables," Sam said, looking up again. "Not just physical ones, but people, their choices."

"You change history and save the world: good," he said. "Or you save the world by stopping yourself from changing history. And how do you know which is which? It's like juggling eggs."

"Stay home, do nothing, be a good little girl," Sam said, stabbing the last piece of tomato.

The Doctor smiled, and his eyes twinkled. "You would never do that, Major Carter. You hunger for knowledge, it burns in you like a star." He tilted his head. "And saving the world, I bet you've done that too."

"It's not a bet I'd take, but for the sake of argument, suppose I had," Sam said. "The world always needs protectors. And I'd like to save it if I could. But I would like to know what I was doing, wouldn't I? Preferably in advance."

He leaned forward, eyes dark in his face. "You can't depend on certainty. There won't be any certainty for you depend on; when the time comes, it'll all depend on your judgement. There won't be anything else. But it'll be enough."

"Is it really that easy?"

"No, it never is," he said. "But that's what you signed up for in the first place, isn't it, Major? It's just a different perspective, that's all." He took a sip from his teacup, and made a face. "And my tea's gone cold."

She smiled. "That's the price of a fascinating conversation."

He brightened. "Quite right." He lifted his wine glass. "To fascinating conversations: may they surprise us with their frequency."

She laughed. "I'll drink to that." She clinked her glass with his. And her heart eased, just a little bit.

**~finis~**

* * *

**Author's Notes**

I'm not sure that I really did quite what the requester wanted, but, hey, once Sam and the Doctor got talking, this was what they wanted to say.

Wikipedia helped me find the event that was the setting for this conversation: on 22nd of May, 2001 was the discovery of trans-Neptunian object 28978 Ixion (Wikipedia: 28978_Ixion). I hypothesized that there would likely be a conference on trans-Neptunian objects, a couple of months later, that Sam Carter would attend. And the Doctor would be interested in such a conference too, if he was passing through.

Yes, Stargate and Doctor Who aren't actually in the same universe, but, well, maybe the Doctor landed in a parallel universe without realizing it.

**Additional Author's Note:**

In regard to the science of this, a few people have pointed out what they thought were errors in the section where the Doctor and Carter were discussing the albedo and mass of Ixion.

Yes, the albedo isn't _directly_ related to the mass, but the albedo does help in trying to figure out both the substance it is made of, and its diameter, based on its reflectivity. And the diameter and composition of the planetoid obviously has a lot to do with its mass!

Carter's objection was that a higher albedo would indicate a higher proportion of water-ice in the planetoid, which wouldn't make sense, because the spectroanalysis showed no water-ice. What the Doctor pointed out was that water-ice wasn't the only substance which had a higher reflectivity, and suggested an alternative (tholin).

Unfortunately, I couldn't spell it out, because they would hardly be explaining things to each other that both of them knew; indeed, the point was that they were enjoying being able to have a high-flying science discussion _without_ having to explain what they were talking about.

If you don't believe me, go and read the Wikipedia article about Ixion; that's where I got the information.


End file.
